1. The Field of the Invention
The present invention relates to optical fibers. More particularly, the present invention relates to multiple mode optical fibers that discriminate against lower order modes such that modal dispersion is reduced and travel distance is increased.
2. Background and Relevant Art
Most optical fibers have the same general structure. The center portion of the optical fiber is the core, which is surrounded by cladding material or a cladding layer. The cladding layer is enclosed in a protective casing. The core of the optical fiber has a refractive index that is higher than the refractive index of the cladding material that surrounds the core. The difference between the refractive index of the core and the refractive index of the cladding material ensures that the light travels through the core of the optical fiber. The optical fiber is effectively a waveguide for light signals or rays.
In optical fibers, the relative sizes or dimensions of the core and of the cladding can vary widely. However, the number of modes that are supported by an optical fiber typically increases as the diameter of the core increases. Thus, optical fibers with small cores support few modes and are often referred to as single mode fibers because they only support one mode, while optical fibers with larger core diameters can support multiple modes. A standard diameter of the core and the cladding material is 125 micrometers, although other sizes are available. However, the diameter of the core will vary and may depend on whether the optical fiber is a single mode optical fiber or a multimode optical fiber.
There are two general types of optical fibers: step index optical fibers and graded index optical fibers. A step index fiber has a definite boundary between the core of the optical fiber and the cladding material or layer where the refractive index suddenly changes. Each mode supported by a step index fiber has its own characteristic velocity or propagation constant. For this reason, pulses of light tend to expand or spread out as they travel through the optical fiber, a phenomenon known as modal dispersion. The modal dispersion is related to both the number of modes supported by the optical fiber and the length of the optical fiber. The modal dispersion increases as the number of supported modes increases and also increases in longer optical fibers. Modal dispersion is also affected by vibrations, temperature, and the like. A practical implication of modal dispersion is that the information encoded in the optical signal can be lost. In particular, low order modes, which typically travel faster, can overtake the slower high order modes that were transmitted at an earlier point in time. As a result, the distances with which conventional multimode optical fibers can be used have been limited.
Because modal dispersion limits the data capacity of step index optical fibers, graded index multimode fibers have been developed. In a step index multimode fiber, there is an abrupt change in refractive index at the boundary between the cladding and the core. A graded index multimode fiber, in contrast, does not have a sudden change in the refractive index. Rather, the refractive index gradually decreases away from the center of the optical fiber. Instead of internal reflection, the light traveling in a graded index multimode fiber is guided by refraction.
In both step index multimode fibers and graded index multimode fibers, light rays follow different paths and occupy different modes. The advantage afforded by the graded index multimode fiber is that light rays that are furthest away from the center of the core travel at a greater velocity than rays that are closer to the core. The difference in velocity is not large, but it is sufficient to compensate for the longer distance that is traveled by the light rays that are furthest from the core. Modal dispersion, the difficulty in fabricating a graded index multimode fiber, and other problems such as mode partition noise limit the effectiveness of graded index multimode fibers in high-speed multimode links. Graded index multimode optical fibers are also costly.
An 850 nanometer multimode fiber communication system that uses step index multimode optical fibers, for example, is a modal dispersion limited system that limits the travel distance of light waves to 220 meters at 1.25 Gigabits per second. The travel distance can be improved using graded index multimode optical fibers, but the systems is still subject to modal dispersion as well as limited by the inherent imperfections that exist in graded index multimode fibers and that limit the travel distance.